ABSTRACT Post-thrombotic syndrome (PTS) is the most common sequelae from deep vein thrombosis (DVT), characterized by vein wall fibrosis, valve destruction, and often occlusion. It is estimated to occur in ~40 ? 50% of those suffering a DVT. No direct medical therapy exists to treat PTS, highlighted by the recent failure of graded compression stockings to prevent PTS (SOX trial). The ATTRACT trial suggested that even those patients treated with thrombolysis still have a ~40% incident PTS at 2 years, and invasive pharmaco- mechanical thrombus removal did not improve outcomes. Significant bleeding risks remain even with the new non-vitamin-K antagonists. Monocyte/macrophages (Mo/M?) are the primary leukocyte directing two key pathobiologic processes: venous thrombosis resolution and the associated vein wall fibrotic injury. Mo/M? are classified by their inflammatory or anti-inflammatory functions, which is a dynamic process in vivo. For example, interleukin-1 (IL-1), IL-12 secreting and cell surface Ly6Chi, CCR2++, CX3CR1+ antigen expression characterizes classically activated, or pro-inflammatory Mo/M?. Conversely, IL-10 secreting, transcription factor Nr4a1 dependent, and cell surface Ly6Clo, CCR2-, CX3CR1++ antigen expression characterizes alternatively activated Mo/M? with pro-healing and inflammation resolving activities. From published and preliminary data, pro-inflammatory Ly6Chi Mo/M? are involved with early VT, followed by a later transition to Ly6Clo. We show that in a stasis murine model of VT that a pro-inflammatory cytokine milieu exists, that early LyC6hi Mo/M? changes over to a LyC6lo content, and that Ly6Clo Mo/M? may drive both VT resolution as well as vein wall fibrotic injury. However, the mechanism of Mo/M? actions in VT resolution and vein wall injury, as well as whether this is thrombogenic model, sex and age dependent is not known. Our overall hypothesis is that VT resolution and vein wall fibrotic injury is dependent on Mo/M? actions, is dependent on model, age, and sex, and can be ameliorated with increasing Ly6Clo Mo/M? in the thrombosed vein. We will address this hypothesis by three specific aims. Specific Aim 1: To define the local environmental and cellular factors that drive Ly6Chi and Ly6Clo Mo/M? phenotypes in the thrombosed vein, with sex, age, and thrombogenic model variation. Specific Aim 2: To directly determine the Mo/M? mediated mechanisms of VT resolution and vein wall injury. Specific Aim 3: To determine if targeted Mo/M? polarization within the thrombus environment can promote VT resolution and vein wall healing. Murine models of VT, with variations of age and sex, Mo/M? conditional deleted and transgenic mice and molecular, immunological, and in vivo imaging will be used to accomplish these aims. The proposed experiments herein will significantly move the field forward by defining the mechanisms of Mo/M? mediated actions in VT resolution and vein wall injury, and will test novel assessments and agents to increase pro-healing Mo/M? activity with potentially translation to PTS therapies.